Microelectronics, including semiconductors, storage devices, and flat panel displays, are generally fabricated in successive layers using photolithographic techniques for patterning surface features. A reticle or mask having a predetermined pattern is evenly illuminated and projected onto a layer of photoresist on the surface of the microelectronic substrate. Exposed portions of the photoresist are chemically altered, rendering them more or less soluble to a developer that removes the soluble portions leaving a positive or negative image of the mask.
High resolution of the surface features is, of course, important; and improved resolution is continually sought for making the surface features smaller and more closely spaced so the resulting electronics can be made smaller, faster, and cheaper. A resolution dimension "R" representing minimum feature size is related to light wavelength ".lambda.", numerical aperture "NA", and a process related constant "K.sub.1 " as follows: ##EQU1##
Feature size "R" can be reduced by reducing the wavelength ".lambda." or the process constant "K.sub.1 " or by increasing the numerical aperture "NA". In production environments, process constants "K.sub.1 " equal to 0.7 to 0.8 are typical, whereas constants "K.sub.1 " as low as 0.5 can be achieved in laboratory settings. Numerical aperture "NA" and wavelength ".lambda." are also related to depth of focus "Df" as follows: ##EQU2##
A depth of focus "Df" of at least a fraction micron (e.g., 0.5 microns) is needed to accommodate flatness variations of the microelectronic substrates and their successive layers. Since numerical aperture "NA" is raised to a higher power than wavelength ".lambda." in the above expression for depth of focus "Df", resolution improvements achievable by enlarging numerical aperture "NA" are much more limited than those achievable by shortening the wavelength ".lambda.".
Wavelengths less than 300 nanometers (NM) can be practically transmitted by only a few optical materials such as fused (synthetic) quartz and fluorite (calcium fluoride). The transmissivity of even these materials deteriorates at wavelengths in the deep ultraviolet range less than 200 NM so a minimum number of optical elements is desirable.
Although it is advantageous to minimize feature size of the images projected onto the microelectronic substrates, the feature size of the masks should remain large enough to manufacture efficiently and to avoid errors from mild levels of contamination. For example, it is important that small specks of contamination do not bridge features of the masks. Mask size can be maintained by optically reducing the projected image of the mask with respect to the mask itself.
Laser light sources operating within the ultraviolet and deep ultraviolet ranges emit light within narrow bands of wavelengths. However, even narrow bands of wavelength cause significant chromatic aberrations in single-material lenses with finite focal lengths. On the other hand, limiting laser output to a single wavelength is inefficient. Accordingly, catadioptric imaging systems have evolved which use reflective optics (mirrors) to reduce image size in combination with refractive optics (lenses) to compensate for symmetrical aberrations of the reflective optics.
Beamsplitters or partially reflective mirrors are used to separate light traveling to and from the reflective optics. Beamsplitters and partially reflective mirrors, particularly when subjected to angularly diverging beams, introduce additional aberrations requiring correction. The beamsplitters also add to the complexity of the imaging systems by misaligning the object and image planes.
A typical catadioptric optical reduction system used for microlithographic projections is disclosed in U.S. Pat. No. 5,241,423 to Chiu et al. A concave spherical mirror provides a four to five times reduction in the projected image size with respect to a mask, and a beam-splitting cube separates light beams traveling to and from the mirror. Groups of refractive optical elements located on opposite sides of the beam-splitting cube toward both the reticle (mask) and the substrate correct for aberrations of the mirror and beam-splitting cube.
Chiu et al.'s reduction system is intended for operation at wavelengths of about 248 NM produced by a KrF excimer laser. However, the large number of refractive elements and the bulky two prism construction of the beam-splitting cube limit usefulness of this system at shorter wavelengths. The transmission of light through fused quartz or fluorite diminishes with shortening wavelengths, so the number and bulk of refractive optics must be limited to utilize wavelengths within the deep ultraviolet spectrum at less than 200 NM length.
U.S. Pat. Nos. 5,251,070 and 5,289,312 to Hashimoto et al. also use a concave mirror to provide most of the reducing power but use a semi-transparent mirror on a plane parallel plate instead of a beam-splitting cube to separate light beams traveling to and from a concave mirror. The former patent of Hashimoto et al. incorporates plane parallel refracting plates to correct aberrations caused by the semi-transparent mirror. The latter patent of Hashimoto et al. uses high-power refractive optics to collimate the beam transmitted through the semi-transparent mirror. This reduces aberrations from the semi-transparent mirror but still requires other refractive optics to counteract aberrations introduced by the high-power refractive optics.